As techniques for detecting a torsional amount between two relatively-rotatable shaft sections, there have been well known techniques which are characterized by provision of detection apparatus, such as a potentiometer or resolver apparatus, on input and output shafts interconnected via a torsion bar. According to the above-mentioned technique using a potentiometer, a slider is mounted on the input shaft while a resistor is mounted on the output shaft, so that a position of the slider contacting the resistor varies in accordance with variation in a relative rotational position between the input and output shafts to thereby provide an analog voltage corresponding to the relative rotational position. According to the technique using resolver apparatus, separate resolver apparatus are provided on both of the input and output shafts so as to detect a relative rotational amount (torsional amount) between the two shafts on the basis of angle signals output from the two resolver apparatus. Further, as a means for detecting rotational displacement between two relatively-rotatable shafts, there has been developed a noncontact-type torque sensor for power steering which employs an induction coil.
Inventions disclosed in Japanese Patent Application Laid-open Publication Nos. 2000-55610, 2002-48508, 2002-107110 and 2002-310816 were each proposed with a view to avoiding the drawbacks of the above-discussed conventionally-known techniques, and each of these proposed inventions employs a noncontact-type, induction-or variable-magnetic-coupling-type sensor that uses a coil section and magnetism-responsive member (magnetic member or electrically-conductive member).
The conventional technique of the type using a potentiometer would always suffer from problems, such as poor contact and failure, because it employs a mechanical contact structure. Further, because there occurs impedance variation of the resistance due to temperature changes, it is necessary to appropriately make temperature drift compensation. Further, the conventional rotational-displacement detection apparatus, known as the noncontact-type torque sensor for power steering employing the induction coil, is arranged to measure an analog voltage level produced in response to minute relative rotational displacement, so that it would accomplish only a very poor detecting resolution. Further, in addition to the need to compensate temperature drift characteristics of the coil, there is a need to appropriately compensate temperature drift characteristics present in reluctance of the magnetic member that varies magnetic coupling to the coil in response to a changing relative rotational position as well as in eddy current loss of the electrically-conductive member. Furthermore, the detection apparatus in the inventions disclosed in the aforementioned patent literatures each do not constitute a sensor of a sufficiently simplified construction.